Heat and Drought Events Alter Biogenic Capacity to Balance CO2 Budget in South-Western Europe

Heat and drought events are increasing in frequency and intensity, posing significant risks to natural and agricultural ecosystems with uncertain effects on the net ecosystem CO₂ exchange (NEE). The current Vegetation Photosynthesis and Respiration Model (VPRM) was adjusted to include soil moisture impacts on the gross ecosystem exchange (GEE) and respiration (R_ECO) fluxes to assess the temporal variability of NEE over south-western Europe for 2001–2022. Warming temperatures lengthen growing seasons, causing an increase in GEE, which is mostly compensated by a similar increment in R_ECO. As a result, there is a modest increase in the net carbon sink of 0.69 gC m⁻² yr⁻¹ but with high spatial and annual variability. The heatwave of 2022 reduced net carbon uptake by 91.7 TgC, a 26.4% decrease from the mean. The interannual variability of NEE is more influenced by drought in temperate humid regions than in Mediterranean semi-arid regions. These results emphasize the vulnerability of the net carbon sink as drying trends could revert the NEE trends, as it is happening for croplands in the French Central Massif.